The present invention relates to a branched polymer and a process for the preparation thereof, and more particularly to a polymer having a high wet skid resistance, an excellent rolling resistance characteristic and a good processability, a process for the preparation thereof, a rubber composition for tire tread containing the polymer and a tire made from the composition.
In recent years, a demand of saving fuel cost for automobiles is increasing more and more. Especially, characteristics of tires have an important influence upon saving fuel cost, and the improvement thereof has been strongly desired.
The characteristics required for tires are mainly abrasion resistance, wet skid resistance, low heat generation, bending resistance, chipping resistance, groove cracking resistance, and the like. It is necessary that tires have various characteristics in good balance. In particular, it is important from the viewpoint of resources saving and energy saving that energy loss is small.
Among these characteristics, a high wet skid resistance required for steering stability and a low rolling resistance required for fuel saving are particularly important characteristics. In a conventional knowledge, it is recognized that these both characteristics conflict with each other.
Hitherto, natural rubber, polyisoprene rubber, high cis-1,4-polybutadiene rubber and styrene-butadiene rubber have been mainly employed as a rubber for tires, particularly as a rubber for tread. Natural rubber, polyisoprene rubber and high-cis-polybutadiene rubber have the feature that the energy loss is a little, thus the rolling resistance is low, but have the drawback that the wet skid resistance against a wet road is low. On the other hand, styrene-butadiene rubber has a high wet skid resistance, but is not sufficient as a material of energy saving tires because the heat generation characteristic is high and the energy loss is large, thus the rolling resistance is high.
In order to remedy the drawbacks of these polymers, a polymer blending technique has hitherto been developed. For instance, a blend of styrene-butadiene rubber and a high-cis-polybutadiene is mainly used in tires for small passenger cars. However, with respect to maintaining a high wet skid resistance and a low rolling resistance, this blend is apart from the level demanded in recent years.
In recent years, a composition containing a diene rubber having unsaturated pendant groups attracts attention, and for instance, it is disclosed in British Pat. No. 1,261,371 and Japanese Unexamined Patent Publication Nos. 62248/1979 and 12133/1980 that such a composition provides tires having improved both wet skid resistance and rolling resistance characteristics.
The rolling resistance of tires is caused by energy loss attended on repeated deformation of a tire at the time of travelling of cars. That is to say, the tire causes compressive deformation, bending deformatin and shearing deformation by load, and at the time of travelling, these deformations continuously move along the circumference of the tire. At that time, the dynamic energy loss occurs during repetition of various deformations and restoration in each portion of the tire. In other words, rubber-like materials, including tires, are the so-called viscoelastomers, and the stress-strain relation is not linear and the strain against the stress shows a following time lag. Owing to such a viscoelasticity of tires, the deformation at the time of touching the ground and restoration at the time of untouching are repeated during travelling, and at that time, the phase difference is caused between the stress and the strain. As a result, hysteresis loss, namely energy loss, is caused. The improvement of the rolling resistance is no more than reduction of such an energy loss. It is known that such an energy is largely affected by a tread rubber compound in addition to structures of carcass and breaker of a tire and a side wall rubber compound. Thus, for reducing the rolling resistance of a tire from the viewpoint of a tread rubber compound, it is necessary to reduce the energy loss owing to compressive deformation, bending deformation and shearing deformation. Considering from the dynamic viscoelasticity of a rubber, this means reduction of loss compliance [E"/(E*).sup.2 ] and loss modulus (E").
On the other hand, the wet grip characteristic is considered to be a frictional resistance which generates against the stress that tires receive from a road upon gliding on uneven road. That is to say, a viscoelastomer such as a tire shows deformation and restoration with a time lag against the receiving stress, and as a result, a torque in the reverse direction to the travelling direction is generated. The resistance due to this torque is a frictional resistance, and it depends on the loss tangent (tan .delta.=E"/E') of the dynamic viscoelasticity (Nippon Gomu Kyokaishi, 48, No. 11, 1970). Therefore, though both the wet grip characteristic and rolling resistance characteristic depend on the dynamic loss characteristics [E", E"/(E*).sup.2 and E"/E'], the wet grip characteristic is raised by increase of these values in contrast with the rolling resistance characteristic. In a word, the dynamic loss characteristic values are desirable to be large for the wet grip characteristic and desirable to be small for the rolling resistance characteristic. Therefore, it has been recognized that these both characteristics conflict with each other, and it has been considered that no satisfactory rubber composition cannot be obtained from the same raw material.